Effect of Potassium Sodium Tartrate on Corrosion Resistance of Phosphating Film on Magnesium Alloy

To improve the corrosion resistance of chrome free phosphating films on magnesium alloy, potassium sodium tartrate was added into the phosphate-permanganate solution. The morphology and chemical composition of the phosphating films were observed and tested by SEM and EDS respectively. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) and polarization curves. The results show that the phosphating film obtained from the solution with potassium sodium tartrate was more compact and had less cracks, and higher film resistance and charge transfer resistance, and the corrosion paths were reduced. Moreover, the amounts of Mn, O and P in the phosphating film increased, and the concents of magnesium phosphate and manganese oxide in film were also higher, and thus the corrosion resistance of the phosphating film was improved.